One DD was 8 months on one trip, and my DS was 10 months on one trip. It was super. They can ride everything (except the obvious/rides with height restricitons, etc.) and go to all the shows, etc. But all the basic rides, like It's a Small World, Winnie the Pooh, etc. are fine. There is plenty to do!
It's not overwhelming. Check out the baby centers in each park. They are awesome and they sell anything you could need (forgottena t the roome, etc.) including diapers, pacifiers, bottles, baby food, etc. They are clean comfortable and cool. You can change or nurse your baby there or get anything you need.
The Poly will be excellent because you can bring your stroller right onto the monorail easily, without having to fold it up.
Oh, and if a character wants to hold your baby, he or she will sit downon the floor first and then hold out his/her arms. It's so sweet. At first I didn't understand why Pinocchio was all of a sudden getting down ont he floor, but when I realized he wanted to hold my baby, then it made sense....nice and safe...and great for photos!
Hope that helps!
Enjoy!!